A slap-hammer is a device that is used primarily in auto body shops to help repair dented body panels and the like. This type of hammer has an elongated body with a self tapping screw at the bottom. A heavy weight slides freely on the body between a lower stop and an upper stop, the former being above the screw and the latter being usually at the top of the device.
When a panel dent is to be "pulled out," a small hole is drilled into the panel in the deepest portion of the dent. The screw is introduced into the hole and the body is rotated, allowing the screw threads to get a firm purchase on the panel. Then, the weight is gently pushed to the lower stop. It is then grasped firmly and pulled rapidly towards the upper stop. The momentum acquired by the weight in travelling is transferred to the upper stop upon collision, allowing a large force to be excited on the panel to pull the dent out.
Frequently, dents are so large that this operation must be repeated. In practice, this can be rather time consuming, because it takes time to rotate the hammer enough so that the user is sure that the screw has a firm purchase on the body panel. After this rotation has been accomplished, the rest of the operation goes relatively quickly.